If you have already decided you want to propose, don't wait till Christmas, that's just too long! She's already been waiting five years.
Some tips on ring shopping if you do decide to go it alone and make the whole thing a surprise (which I highly recommend):
Most girls like white gold these days, unless she has traditional taste or has mentioned that she likes yellow, I would go with a white gold band.
Diamonds are priced based on the carat (which is the weight, not the size) the color, the cut and the clarity of the stone. A really fancy setting will also affect the price. A jeweller will tell you all about this and you will probably start stressing out about the quality of the stone you are buying. Don't. Very few people can look at two different stones the same size and tell you for sure which one is worth more. Just as long as there are no major flaws, just get the biggest looking diamond in the simplest setting you can find. Solitaries are always good.
I don't know about over there but in Canada most jewellers mark up their diamond rings by 300% or more. Try and get an older person to help you (better chance they might be an owner) have a big wad of cash with you and don't ever pay the tag price. Be prepared to leave if they won't give you a good deal.
One last idea which you may or may not want to consider. Look around some pawn shops for rings and see if you can find a nice diamond for a good price (personally, I would only buy one with an appraisal), then take it to a goldsmith and have the stone removed and put in a brand new ring. Might be a way to get her something really nice without spending a fortune. If you do this though, take it to your grave. Tell her you had the ring custom made, but don't tell her the diamond is second hand.
One last thing, my ring was only $375. I picked it myself and purposely went with something small because we had no money at the time. I couldn't care less, I just was so in love and wanted to be married. Then again, I only had to wait four months for that moment. The impression I have got from many of my girlfriends is the longer the wait for a proposal, the bigger the expectation is going to be.
Hope this helps!